Before integrated circuits can be marketed, they must be tested in order to insure accurate and complete operation. Many methods are utilized for testing integrated circuits, including initial DC testing of the wafer, and then testing the wafer at the AC level including testing the circuit functionally. Those integrated circuits on the wafer that pass these initial tests are then packaged in the standard industry dual-in-line or other packages and undergo even further testing. Many integrated circuits will be needlessly packaged, however, because prior art testing at the wafer level is inaccurate due to the high noise level in the output test signal from the DUT. Unfortunately, if an integrated circuit device is defective, then much expense has been expended in packaging the integrated circuit chip and including it in the testing process.
For highly sensitive devices, such as charge coupled devices (CCD), it may be simpler and less expensive to test the DUT prior to packaging or at least prior to completion of the packaging when the chip is sealed within an integrated circuit housing.
Commercial machines are available for testing at this level, also, and provide for apparatus upon which the chip may be applied, and with external probe pins, apply signals to and receive output from the integrated circuit chip at predesignated pads on the integrated circuit chip. The probe cards, as they are known by, are akin to printed circuit boards with edge fingers so that the cards can be interchangeable with other cards to test different integrated circuit devices. These probe cards are typically two layers fabricated with a fiberglass or phenolic material. These cards do not incorporate any type of ground plane or central ground system. Many cards are designed to use jumper wires which parallel each other and are long in length. Unfortunately, many or all of these factors contribute to the inherent noise of the conventional probe card. Obviously, increased noise to he output of the system detracts from the testing process of the integrated circuit chip itself, as the DUT output signal, being inaccurate, does not truly depict the accuracy of the signal generated by the integrated circuit chip itself. The output signals from the device under test (DUT), in typical prior art testing apparatus, must travel through the blade connection to the probe card and on through long jumper wires to the edge connector onto the drive board where it finally connects to a high capacitance cable between the driver board and the measuring test instrument. The loading of this signal path causes tremendous degradation of the output signal, thus accurate measurements of this signal from the device under test are impossible.
According to the present invention, a unique customer active wafer probe card is utilized to test an integrated circuit device such as a charge coupled device. The card is unique in that all clock drivers would be located on the probe card itself, with the driver output tied directly to the probe element to reduce the noise and distortion effects caused by long cable lengths. All dc lines would have decoupling capacitors located on the probe card. The ac and dc signal paths would be isolated by a third layer ground plane placed between the ac and dc layers. The edge card connectors would be eliminated and standard berg type connectors would be used instead. The output signal would be buffered on the probe card to reduce the effects of loading caused by the patch cable and the measuring instrument. Accordingly, the DUT may bypass much of the prior art wafer level testing and proceed from testing at the DC probe level directly to the testing after packaging because most inherently defective integrated circuits will be uncovered by the technique and apparatus discussed herein .